INVESTIGATION ON WEELDABI ITY OF HSLA4I STEEL UNDER SUBMERGED ARC WELDING PROCESS

Abstract

The influence of welding energy input on the mechanical properties such as tensile, charpy impact toughness, bend and microhardness and on microstructure was investigated for base metal, weld metal and heat affected zones [0.5mm-1.5mm from fusion line) of double pass submerged arc welds deposited in 12mm thick HSLA-80 steel plates over an energy input range of 2-4 KJ/mm, keeping the wire-flux combination constant throughout the investigation. The microstructure of weld deposit appears to consist of acicular ferrite and bainite. Coarsening of ferrite is observed with the increase in the energy input. In HAZ, bainite content increased and the ferrite content reduced with the increase in energy Input. At lower energy inputs, the weld metal microhardness exceeded the corresponding HAZ microhardness. But this inequality was reversed at higher energy inputs. The microhardness of the weld metal decreased slightly with the increase in the energy input. Also, the microhardness of the weld metal, has been found to be comparatively higher than that of the base material, at all energy inputs. The microhardness of the HAZ does not change significantly both at lower energy inputs upto 3.0 KJ/mm and higher energy inputs above 3.5 KJ/mm. But it increase sharply between 3.0 and 3.5 KJ/mm. Also the hardness in the HAZ becomes iii low in the regions close to the fusion and the distance of this low hardness region increases with the energy input. Under tensile testing, fractures occur in the HAZ close to the fusion line and the corresponding hardness at the fracture location decreases with the increase in energy input. All the mechanical properties i.e., the ultimate tensile strength, yield strength, elongation and reduction in cross sectional area of the weldment were significantly lower than the base material. Also, the ultimate tensile strength and yield strength did not change significantly with energy input. The Cv-toughness of the weld centre is enhanced, but that of the HAZ (0.5-1.5mm from the fusion line) reduces with the increase in energy input. Also, at a given test temperature the Cv-toughness of the weld centre is significantly lower than that the base material, but it is comparable in case of the HAZ, for all energy inputs. The Cv-toughness of the weld centre is found to be more sensitive to test temperature than that of the HAZ. In the bend test, the elongation has been found to be relatively enhanced with the Increase in energy input at,low heat inputs. Also, the bending angle at crack initiation is enhanced with the increase in energy input.